Ok. I play orks, and my usual opponent is
SW..Trust a sneeky deffskull ta blab, huh
Assuming 3 battlewagons as heavies and 45 lootas i'd suggest trying a few of these suggestions, if you have the models.
Long fangs with missile launchers. They are such a staple of the
SW list these days, but they are very effective against orks. Use Krak to sideshot the battlewagons (tiny front and long makes for easy sideshots). Use massed frag fire against the loota blocks. Cause enough damage to force a morale check and watch them run, as they can't take bosspoles and a 15 strong unit will lose out on Mob Rule fearless fast.
Whirlwinds. I know that people see them as a weak option, but they work well vs infantry blocks with paper armour, and get to ignore the wall they are camped behind too..
Vindicators will make almost anything in the ork codex fold. You can take out a whole unit of lootas in one shot, and they'll have a hard time punching your front armour. They will also put big holes in battlewagons, and even an indirect hit can punch rear armour just fine..
Take Njal, if you like. Late in the game he's pretty much instakilling anything his storm or powers hit. I have a tough time with him..
JOTWW is particularly hated by orks and necrons for a reason. Being able to snipe the
PK or warboss from a unit with little chance of a save is nasty.
You could try getting in behind his battlewagon rush with drop pods, and shooting the
AV 10 to all hell

Works pretty well on that list. Give them Deathwinds if you like, or load them out with venerable dreads and walk them into the lootas..
Engage the lootas in
CC. If they're fighting they aren't shooting

Shooting back also works pretty well, as lootas are wearing beercans for armour. Heavy bolters make a mess of them, and a
LR Crusader can absolutely feth them up (sometimes two units at a time due to
POTMS) while being completely immune to their fire. Concentrated bolter volleys also wipe the floor with orks. Pod in a grey hunter pack, unload and blast away
Heavy bolters aren't a cruddy option vs orks. They do jack vs marines, but anything infantry but a meganob on the ork list will go down hard to them. They can also punch rear armour and trukks
Orks have a real struggle with
AV 14. Landraiders are a pain for orks to deal with.
Try a fast moving landspeeder or assault marines as a counter to the lootas. The assault marines (skyclaws?) are quite capable of messing up a loota unit in short order, and their armour will shrug loota fire most of the time (though they will be wounded a lot).
I'll ask my regular foe and see if he can come up with more
Frankly if your opponent is running battlewagon spam with Ghaz instead of 2
KFF meks he's wasting some of the potential there, and 45 lootas puts out a lot of fire but prevents him from using other elite options. Lootas can be dealt with pretty easily, with a little planning